The process of refining glass is the subject of various theories, but practice generally provides for the introduction into the vitrifiable charge of glassmaking raw materials of certain agents, called refining agents, able to give rise to the release of gas in a range of high temperatures, corresponding to slight viscosities of the glass, said gas being able to be reabsorbed by the molten glass at lower temperature levels. The ascending bubbles of gas released by the refining agents particularly have a mixing role for the vitreous mass, and also serve to entrain and remove small bubbles whose ascending speed is very slight and which would otherwise be trapped in the melt at the end of the melting.
It is customary to introduce into the vitrifiable mass, as refining agents, various metal sulfates such as sodium sulfate, gypsum, or other sulfate base products bringing metal oxides into the glass composition. It is generally accepted that these sulfates play a favorable role during the melting phase of the process and that, during the refining phase, release SO.sub.3 inside the various small bubbles thus causing them to grow in size and rise to the surface. This latter result appears to be facilitated by the use of certain reducing agents which might be expected to lower the solubility of the sulfate and cause SO.sub.2 to be evolved.
Therefore, it has become frequent practice to add carbon to certain vitrifiable mixtures containing sulfates. It seems that the presence of carbon facilitates the decomposition of the sulfates and favors the release of SO.sub.2 rather than the direct formation of the more soluble SO.sub.3. According to another hypothesis, the carbon could also reduce, at least in part, the sulfates into sulfides, which would play a favorable role during melting since, by oxidation or reaction with the sulfates, they would liberate SO.sub.2, which could be reoxidized under a layer of surnatant raw materials and thus restore the sulfate to the level where it is most useful to accelerate melting.
Going on this hypothesis, it has also been proposed to introduce sulfur directly into the vitrifiable mixture in the form of sulfides, possibly incorporated in various materials such as blast-furnace slags, by-products of the metallurgical industry, or even frits specially made for this used.
These various improvements each comprise a certain number of drawbacks. Thus, in the processess consisting of introducing metal sulfides in the vitrifiable mixture of glass to be produced, or in a mixture intended for elaboration of an intermediate frit, considerable difficulties appear from the first in preparing the charge, because of the chemical instability, hygroscopicity and toxicity of most of these sulfide salts. Further, during melting, the latter give rise to considerable losses because of their very great vapor pressures.
As far as metallurgical slags are concerned, their use is limited by the lack of consistency in their compositions, and by their great iron content which practically rules out flint glasses, without the costly operation of deferrization.
Of the processes cited above recommending the use of vitrifiable charges containing both metal sulfate and carbon, some show a certain benefit in making their mixture beforehand, in particular the one that proposes the use of ground coke impregnated with a solution of a soluble metal sulfate.
This latter proposal, although an advantageous form of refining material, involves a certain number of constraints, particularly in the choice of the raw materials and in the mode of preparation of the product, and leads to difficulties in use, for example, in avoiding segregation and obtaining a good homogeneity of tint.
The present invention, which makes various improvements in the processes using vitrifiable mixtures containing sulfates and carbon, aims at obtaining results at least at as high a level as that of the processes using sulfides, while avoiding the various drawbacks mentioned above. It applies to glass compositions that are very diverse both in the nature of their components and in their mode of presentation. It brings very advantageous solutions to certain demanding fabrications, such as those of glasses with a low iron content or a high sulfide content.
The invention very particularly facilitates the refining of glass and also makes possible an acceleration of this operation to increase the pull of the furnace and reduce the temperature at which it is performed, and it also reduces the consumption of fuel, wear of refractories, and loss of certain elements, etc.